The deposition of organic and/or inorganic foulants on process equipment accounts for a significant amount of energy loss in oil production, refining and chemicals manufacturing. For example, the thermal processing of crude oils, blends and fractions in heat transfer equipment, such as heat exchangers, is hampered by the deposition of insoluble asphaltenes and other contaminants (i.e., particulates, salts, etc.) that may be found in crude oils. Further, the asphaltenes and other organics may thermally degrade to coke when exposed to high heater tube surface temperatures.
The most common heat exchanger in oil refineries and petrochemical processes is the shell-and-tube type, which consists of a shell with a bundle of tubes inside it. The crude oil runs through the tubes, and the heating fluid flows over the tubes (through the shell) to transfer heat to the crude oil through the tube/shell walls. Typically, the heating fluid is a petroleum fraction with a specific boiling range, which in general comes from a side stream of the vacuum or atmospheric pipe still. If the heating fluid is a heavy (high boiling) fraction, the operating temperature is high compared to that used with a light (low boiling) fraction.
Fouling in heat exchangers receiving hydrocarbon process streams can result from a number of mechanisms including chemical reactions, corrosion, deposit of existing insoluble impurities in the stream, and deposit of materials rendered insoluble by the temperature difference between the process stream and the heat exchanger wall. For example, naturally-occurring asphaltenes can precipitate from the crude oil process stream, thermally degrade to form a coke and adhere to the hot surfaces. Further, the high temperature difference found in heat transfer operations result in high surface or skin temperatures when the process stream is introduced to the heater tube surfaces, which contributes to the precipitation of insoluble particulates. Another common cause of fouling is attributable to the presence of salts, particulates and impurities (e.g. inorganic contaminants) found in the crude oil stream. For example, iron oxide/sulfide, calcium carbonate, silica, sodium chloride and calcium chloride have all been found to attach directly to the surface of a fouled heater rod and throughout the coke deposit. These solids promote and/or enable additional fouling of crude oils. The fouling propensities of different hydrocarbon streams (such as crude oils or refinery process streams) may vary considerably. Under the same operating conditions, some streams foul easily, while other streams may experience minimal fouling.
The buildup of insoluble deposits in heat transfer equipment, such as a heat exchanger, creates an unwanted insulating effect, reduces the heat transfer efficiency and increases energy consumption. Fouling also reduces the cross-sectional area of process equipment, which decreases flow rates and desired pressure differentials to provide less than optimal operation. As a result, heat transfer equipment is ordinarily taken offline and cleaned either mechanically or chemically, resulting in lost production time. In many cases, fouling even causes unwanted and unexpected shutdowns.
Great strides have been made to develop antifoulant agents, additives or coatings. While the addition of antifoulant additives leads to significant energy savings, it introduces attendant costs, including the cost of the additive itself and the cost of removing the additive from the process downstream. As such, it is vitally important to minimize the amount of additive that is introduced to the process while achieving the desired level of fouling reduction, i.e., using only the minimally required level of additive to achieve effective fouling prevention. This requires the development of an accurate and sensitive way of a priori prediction of the fouling propensity of an oil so that an optimal dosage of the additive can be estimated and applied. As well, the fouling propensity can be used as a basis for selecting, purchasing, and blending various hydrocarbon streams to achieve effective fouling prevention.
Furthermore, as fouling is often sensitive to the processing temperature of the hydrocarbon stream, it would be useful to develop a way to determine a priori the temperature sensitivity of fouling by a hydrocarbon, which can not only help predict the fouling propensity of a hydrocarbon but also help select operating conditions so that fouling can be avoided or minimized.